The present invention relates to a transmitting/receiving system used for transmitting a high-frequency field to excite a magnetic resonance (MR) phenomenon and receiving and detecting an MR signal produced by the MR phenomenon in a magnetic resonance imaging (MRI) system for exciting the MR phenomenon in an object and for obtaining MR information in the object.
In an MRI system, a uniform static field is applied to a desired portion of an object, and a transmitting RF coil for forming a high-frequency field, i.e., an RF (radio frequency) field in a direction perpendicular to the static field is used. Thus, an MR phenomenon is caused in a specific slice from which a tomographic image is to be obtained. Then, MR signals generated from atomic nuclei after the RF field is removed are detected using a receiving RF coil. Upon excitation of the MR phenomenon and acquisition of the MR signals, in order to select a specific slice and/or to provide position information to MR signals, a gradient field is applied to the object, as needed.
FIG. 1 shows an arrangement of a conventional transmitting/receiving system in the MRI system.
A transmitting system comprises transmitting/receiving coil 1, variable capacitor 2, RF pulse oscillator 3, and cross diode switch 4. A receiving system comprises capacitors 8 and 15, preamplifiers 9 and 16, and first and second receivers 18 and 19.
Cross diode switch 4 has a basic arrangement in which two diodes are cross-coupled. Cross diode switch 4 serves as a high-frequency switch for passing a signal having an amplitude larger than a predetermined value and cutting off a signal having a smaller amplitude. RF pulses from oscillator 3 are applied to transmitting/receiving coil 1 through cross diode switch 4, and the RF pulses are transmitted from transmitting/receiving coil 1 to an object.
MR signals from the object are received through first or second receiver 18 or 19.
First receiver 18 has transmitting/receiving coil 1, variable capacitors 2 and 7, duplexer 5, and cross diode switch 6. Variable capacitor 7 is adopted for tuning, and is used for adjustment for tuning first receiver 18 with the MR signals from the object. Transmitting/receiving coil 1 is used for both transmission and reception, and normally comprises a saddle coil. Duplexer 5 is provided as needed, and is used for switching transmitting and receiving modes together with cross diode switches 4 and 6. Duplexer 5 is constituted by, e.g., a .lambda./4 cable or coil. First receiver 18 is used for receiving MR signals in a whole body (WB) imaging mode.
Second receiver 19 is constituted by receiving coil 12, cross diode switch 13, and variable capacitor 14 which are connected in parallel with each other. Variable capacitor 14 is used for adjustment for tuning second receiver 18 with the MR signals from the object. As receiving coil 18, a small coil having a good filling factor for obtaining a higher S/N ratio than that of transmitting/receiving coil 1 is adopted. Second receiver 19 is often used for receiving MR signals in a head portion imaging mode. A surface coil used for receiving MR signals in a local imaging mode for locally imaging a portion near a body surface belongs to receiving coil 12.
In the above arrangement, in the WB imaging mode, RF pulses are transmitted through transmitting/receiving coil 1, thus causing the MR phenomenon in an object. MR signals generated by the MR phenomenon are amplified by preamplifier 9, and are then processed by data processor 11. A method wherein excitation of the MR phenomenon and reception of MR signals are performed using only transmitting/receiving coil 1 is called a single coil method.
In the head portion (brain) imaging mode, RF pulses are transmitted through transmitting/receiving coil 1 to excite the MR phenomenon in the same manner as in the single coil method. In this case, MR signals are received through second receiver 19, and the reception signals are amplified by preamplifier 16. Thereafter, the signals are processed by data processor 11. In this manner, a method wherein transmission of the high-frequency field and reception of the MR signals are respectively performed by transmitting coil 1 and receiving coil 12 is called a cross coil method.
However, first and second receivers 18 and 19 are designed and adjusted to resonate, i.e., tuned with the MR signals in order to efficiently receive the MR signals from the object. When MR signals are received by first receiver 18, receiving coil 12 is unnecessary, and second receiver 19 including coil 12 is disabled. When MR signals are received through second receiver 19, transmitting/receiving coil 1 is necessary for transmitting a high-frequency field, and it cannot be disabled. In this case, since first receiver 18 is also adjusted to be tuned and resonate at a frequency of the MR signals, an inductive electromotive force to be induced in only receiving coil 12 is also partially induced in transmitting/receiving coil 1. As a result, a reception level of MR signals of second receiver 19 is decreased. A decrease in reception level of MR signals causes degradation in image quality of an MR image to be formed.